Crumb rubber modified asphalt with enhanced settling characteristics

ABSTRACT

An improved asphalt having enhanced settling properties. The asphalt includes crumb rubber particles which have been treated to produce a greater number of carboxylic sites on the surface of the crumb rubber. These carboxylic sites interact with the functional groups in the asphalt, resulting in a homogeneous mixture having improved separation characteristics as well as the improved rheological properties due to the inclusion of the crumb rubber particles.

STATEMENT OF GOVERNMENT INTEREST

The present invention may be made or used by or on behalf of theGovernment of the United States without the payment of any royaltythereon or therefor.

This is a divisional of U.S. Ser. No. 08/922,605 filed Sep. 3, 1997, nowU.S. Pat. No. 5,851,276.

BACKGROUND

The unique chemistry and internal molecular association of asphalt areresponsible for its mechanical properties and hence for its use as abinder in asphalt concrete pavement. Large tonnages of asphalt are usedevery year in constructing roads throughout the world; thus anyimprovement in its properties, no matter how small, translates intoconsiderable cost savings. The present invention is a means and methodof improving the rheological properties of crumb rubber modified asphaltwhich is later used in pavement.

Adding crumb rubber to hot asphalt results in improved properties suchas a wider useful temperature range. However, reports of performance ofprior art crumb rubber modified asphalt concrete vary considerably; somepavements ravel after only a limited amount of use, sometimes there is adecrease in the mechanical properties of the binder, and sometimes thereare inconsistencies in the applicability of the asphalt concrete in thefield. These variations result from differences in the types of asphalt;the source, composition, size, and process of making the crumb rubberparticles; and in the process of incorporating the crumb rubberparticles into the asphalt. One of the primary problems with prior artcrumb rubber modified asphalt binder is that it has poor settlingcharacteristics; that is, the crumb rubber particles tend to settle orto produce new molecular size particulates which tend to settle to thebottom of the tank before the asphalt is mixed with aggregate. Anotherproblem is that the particles do not disperse evenly through theasphalt. This may contribute to the problems noted above, particularlythe ravelling.

It has been found that the quality of the asphalt after the addition ofthe crumb rubber is a function of the amount of interaction between theasphalt and the crumb rubber particles, which in turn is a function ofthe number of carboxylic sites on the surfaces of the particles. Whenthe particles have a large number of carboxylic sites, they mix moreevenly due to interaction with asphalt functional groups and the finalproduct is much smoother; the crumb rubber particles also stay insuspension to a much greater degree than before.

OBJECTS OF THE PRESENT INVENTION

Accordingly, it is an object of the present invention to provide a crumbrubber modified asphalt that has improved rheological properties.

It is a further object to provide such a crumb rubber modified asphaltthat has improved settling characteristics.

It is a further object to provide such a crumb rubber modified asphaltthat can be prepared using readily available materials.

It is a further object to provide such a crumb rubber modified asphaltthat is environmentally benign both in its preparation and its use.

It is a further object to provide a means and method for increasing thenumber of carboxylic sites on crumb rubber that is to be added toasphalt.

SUMMARY

Briefly, the present invention is the creation of carboxylic sites onthe surface of crumb rubber particles that are later added to asphalt.This is done by treating the crumb rubber particles with a carboniumion-generating material such as hydrogen peroxide. The hydrogen peroxidefirst creates carbonium ions on the surface of the crumb rubberparticles, then converts these ions to carboxylic sites. The functionalgroups in asphalt and the carboxylic sites on the crumb rubber particlesinteract with each other. Since the treated particles have morecarboxylic sites than untreated particles, there is a greaterinteraction between the asphalt and the treated particles. This producesa crumb rubber modified asphalt that has the desired rheologicalproperties; i.e. has an increased useful temperature range over thatmade with untreated crumb rubber particles.

IN THE DRAWINGS

FIG. 1 shows the improvement in the rheological properties of asphaltsAAD-1, ABM-1, and AAM-1 using the present invention.

FIG. 2 shows a possible mechanism for the creation of the carboxylicsites on the crumb rubber.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Prior art crumb rubber modified asphalt is made by simply adding crumbrubber to asphalt. While this increases its operating temperature range,the asphalt is subject to ravelling and other problems. These effectsare thought to be at least partially the result of the fact that thecrumb rubber particles tend to form clumps and settle out of the asphaltwhile it is still in the melting tank before being added to aggregate.

The present invention alleviates this problem by treating the crumbrubber particles so that the asphalt interacts with them more than itdoes with untreated particles. When the treated particles are added tothe asphalt they are more fully dissolved in and suspended in theasphalt, thereby reducing the problems which are thought to be a resultof the settling out of the particles in the prior art crumb rubbermodified asphalt.

The crumb rubber particles of the present invention are treated with acarbonium ion-generating material such as hydrogen peroxide. Hydrogenperoxide first creates carbonium ions at the carbon black by utilizingits oxygenated sites on the surface of the particles, then convertsthese carbonium ions to carboxylic sites. The treated particles thushave more carboxylic sites than untreated prior art particles. Thefunctional groups in the asphalt then interact with the carboxylicsites, thereby causing the treated crumb rubber particles to dissolve inand be suspended in the asphalt to a much greater degree than untreatedparticles. This results in an increase in the useful temperature rangeover and above that which is obtained from adding prior art untreatedparticles to asphalt and a decrease in the ravelling and other problemsnoted above.

It is desirable that the crumb rubber particles be as small as possibleso that they are most easily dissolved in and suspended in the asphalt.The particles used in the examples were all -80 mesh size and 15% byweight of the asphalt. Since crumb rubber is more expensive thanasphalt, 15% is the maximum that is presently used in crumb rubbermodified asphalt. Therefore that is the amount that was used in theexamples, although it may be possible to use more as a result of theincreased interaction of the asphalt with the improved particles of thepresent invention.

The particles were treated by mixing them with hydrogen peroxide at atemperature of about 65-85 degrees Centigrade while stirring for about20 to 30 minutes to produce a freely-flowing powder. They were thensprinkled on the surface of the asphalt at a temperature of about143-183 degrees Centigrade after which it was stirred for about one anda half hours while maintaining the temperature. For AAM-1 asphalt theamount of hydrogen peroxide was 0.035 millimoles of peroxide per gram ofasphalt; for AAD-1 it was 0.025 millimoles per gram; and for asphaltABM-1 it was 0.040 millimoles per gram. These amounts wereexperimentally determined and are optimized to produce the best results.The optimum amount of hydrogen peroxide is a function of the polarcompounds and functional groups in the asphalt; it will generally fallwithin the range of about 0.02 to about 0.06 millimoles per gram ofasphalt.

FIG. 1 shows the improvement in rheological properties of the modifiedasphalt of the present invention. As can be seen, unmodified grade AAD-1asphalt has a temperature range of -31 degrees Centigrade to 63 degreesCentigrade; the addition of prior art untreated crumb rubber particles(i.e. the control sample) increased the range to -28 degrees to 80degrees, while the addition of particles treated according to thepresent invention increased the range to -33 degrees to 81 degrees. Thusuntreated crumb rubber particles increased the high temperature 17Centigrade degrees, a definite increase in performance at the high end,but also increased the low temperature 3 Centigrade degrees, a decreasein performance at the low end. When the particles are treated accordingto the present invention, the high temperature is raised 18 Centigradedegrees and the low temperature is lowered 2 Centigrade degrees--animprovement in performance at both ends of the temperature range.

Asphalt AAM-1 shows improvement in both high and low temperatureperformance with the addition of untreated and treated particles.Untreated particles raise the high temperature 2 Centigrade degrees, andtreated particles raise the high temperature an additional 3 Centigradedegrees; untreated particles lower the low temperature 7 Centigradedegrees, and treated particles lower the low temperature an additional 3Centigrade degrees. Thus the high and low temperatures of asphalt AAM-1are improved more through the addition of crumb rubber particles thathave been treated according to the present invention than through theaddition of prior art untreated particles.

Asphalt ABM-1 also shows improvement in both high and low temperatureperformance with the addition of treated particles. The high temperaturewas raised 6 Centigrade degrees and the low temperature was lowered 5Centigrade degrees, for an overall increase in temperature range of 11Centigrade degrees versus an overall increase in temperature range of 6degrees Centigrade for untreated particles.

Although the exact mechanism is not known at the present time, FIG. 2shows a possible mechanism for the creation of carbonium ions and theirsubsequent conversion to carboxylic sites.

I claim:
 1. An improved crumb rubber particle for addition to asphaltcomprising crumb rubber particles that have been treated with acarbonium ion-generating material.
 2. An improved crumb rubber particleas in claim 1 wherein said carbonium ion-generating material compriseshydrogen peroxide.